mbox: Use MACHINE_FEATURES for virtual-pnor

Define MACHINE_FEATURE openpower-ubi-fs to enable the
virtual-pnor functionality and enable on Witherspoon.

Change-Id: I57a6a39cf36501cf6c638df13b4e4b08a70fec26
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-witherspoon/conf/machine/witherspoon.conf b/meta-witherspoon/conf/machine/witherspoon.conf
index 62aca43..c253bf0 100644
--- a/meta-witherspoon/conf/machine/witherspoon.conf
+++ b/meta-witherspoon/conf/machine/witherspoon.conf
@@ -3,7 +3,7 @@
 
 UBOOT_MACHINE = "ast_g5_ncsi_config"
 
-OBMC_MACHINE_FEATURES += "obmc-mrw"
+OBMC_MACHINE_FEATURES += "obmc-mrw openpower-ubi-fs"
 
 PREFERRED_PROVIDER_virtual/p9-vcs-workaround = 'vcs-pre-post'